Metal sheet coated with a thermoplastic resin on one or both sides has long been used for shaping into bodies and end panels for food and beverage containers. The coating is applied by a variety of processes including roll coating, reverse roll coating, spraying, electrocoating, powder coating, and lamination. Coatings are applied to the metal sheet in order to improve corrosion resistance, formability, and appearance.
Electron beam radiation is used in the prior art to cure a coating material after it has been applied to a substrate. “Curing” is a common name for the process in which an organic coating applied to a substrate is polymerized to or nearly to the maximum extent practical, and in addition, curing drives off of all or nearly all of the solvent if any is present. The curing of a coating may involve polymerization from monomers, or the chain lengthening (cross linking) of oligomers or polymers, or any combination thereof. The average molecular weight of the polymer chains is increased, and any solvents which might be present are largely driven off. Electron beam curing may be defined as the above curing process induced by electron beam irradiation. Solvents are frequently not present in electron beam cured formulations, and therefore are frequently not driven off in the curing process. In electron beam curing, the electron beam ionizes specific molecules, which initiate a free radical reaction in the uncured coating and causes polymer cross linking (increasing average polymer molecular weight). A typical example of electron beam curing is in U.S. Pat. No. 4,452,374, granted to Hitchcock et al. In this patent, the polymer coating is cured by “cross linking the (polymer) via the use of conventional high energy electron beam irradiation techniques”. Many other examples exist in the patent literature and published references. However, the curing treatments are stopped when full polymerization is reached or approached. This is because added energy, whether thermal, electron beam, or other, will begin to break polymer bonds, shorten chain length, reduce average molecular weight, and deteriorate the coating properties if energy application is continued beyond the point of full polymerization. Such property deterioration may include embrittlement, loss of strength, adhesion failure, and or yellowing of the polymer coating. With no nearby reactive materials present to continue the polymerization process, application of added energy causes the polymers to simply undergo a scission process in which chains are shortened and average molecular weight decreases.
Polymer coatings which are applied as fully polymerized materials do not receive a curing step after they are applied to a metal substrate. In extrusion coating, a polymer which has already been fully polymerized is melted and extruded onto the metal surface. A small amount of additional heating may be used to assure that the molten polymer flows into microscopic porosity in the substrate, but no further polymerization takes place, and no additional heating or irradiation is applied to cure the coating, drive off solvents, or further polymerize the coating molecules. In powder coating, fully polymerized powdered polymers are applied to the substrate, and then heated to melt them into a coherent mass and induce adhesion to the substrate. Again, however, no further polymerization takes place, and no heating or irradiation is applied to cure the coating, drive off solvents, or further polymerize the coating molecules.
Coating processes most commonly used commercially at the present time require solvent based systems that generate vapors. In order to provide a more environmentally acceptable coating process not requiring any solvents, extrusion coating of thermoplastic polymers onto metal surfaces has been proposed. Some prior art patents disclosing extrusion coating onto metal include Smith et al. U.S. Pat. No. 5,407,702; Levendusky et al. U.S. Pat. No. 5,919,517; Hitchock et al. U.S. Pat. No. 4,452,374; and Schmid et al. U.S. Pat. Nos. 5,942,285 and 6,153,264.
Extrusion coated aluminum alloy sheet performs adequately for some purposes. However, the highly ductile nature of thermoplastic polymers leads to problems when the polymer coated sheet is formed into end panels for food and beverage containers. The first problem, called “feathering” or “membraning”, occurs when the thermoplastic coating separates from the metal when a pull tab or stay on tab is opened. A thin sheet of the separated coating is visible, either partially covering the container spout or sticking out from the opened panel. A second problem is called “angel hair”, in which polymer coating pushed into the score line on the end panel forms fine filaments when the end is opened, thereby creating an undesirable appearance. The fine filaments of coating are free to enter the container through the opening or to enter the consumer's mouth. Angel hair sometimes also forms at the peripheral edges of polymer coated metal disk blanks for can bodies and at the top edges of polymer coated can bodies.
Accordingly, there still remains a need to provide polymer coated metal sheet having improved resistance to feathering and angel hair formation when the coated sheet is shaped into container end panels.
A principal objective of the present invention is to provide a process for making a metal-polymer composite including a polymer coating having scissioned polymer chains in order to improve resistance to feathering and angel hair formation.
Some advantages of the invention are that the process is inexpensive, can be performed quickly, and is capable of being operated without generating pollutants.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.